Methods are available to cryogenically freeze food items. Some of the benefits of cryogenic cooling and freezing include short cooling/freezing times, inhibition of microbiological activity and improved food quality due to decreased ice crystal formation and dehydration. The most common methods for cryogenic freezing involve spraying the surface of the food items with nitrogen (N2) or carbon dioxide (CO2). In N2 systems, liquid N2 is typically sprayed into the freezer were it contacts the food items. As droplets touch the food item, the liquid changes to a vapor, extracting heat from the food surface in the process. The vapor distributed within the freezer can also be driven by convective currents to increase the freezing or cooling rate. Typically, about half of the cooling effect is provided by the N2 phase change from liquid to vapor.
Different types of cryogenic freezers are available. These include immersion freezers, tunnel and spiral freezers, and impingement freezers. Immersion freezers immerse the food item in a liquid bath, which rapidly freezes the item and forms a “crust” on the product which reduces the dehydration rate and clumping of the food item. This method leads to significant thermal shock and is thus not suitable for delicate food items. Tunnel freezers transport the product through the freezer where a liquid cryogenic substance is sprayed onto the product. Spiral freezers are similar, except that they use a vertical axis spiral belt rather than a straight belt to convey the food items, and therefore require less space on a production floor. Cryogenic impingement freezers use high velocity air jets driven onto the food item to freeze or cool the food items and decrease the boundary layer resistance to heat transfer, providing a relatively higher rate of heat transfer. However, conventional impingement jets only reach maximum velocities of about 20 meters per second. Cryogenic impingement freezers typically lead to higher production rates, but have a higher capital cost.